1. Field of the Invention
This invention relates to new polyureas modified by neutralized N-cyanaminocarbonyl urea groups and to a process for their production.
2. Description of the Prior Art
Aqueous solutions or dispersions of anionically modified polyisocyanate addition products, more particularly aqueous dispersions of anionically modified polyurethanes or polyurethane ureas, and their use for the production of coatings are known [cf. for example DE-PS 1 184 946, DE-PS 1 178 586, DE-AS 1 237 306, DE-OS 1 495 745, DE-OS 1 595 602, DE-OS 1 770 068, DE-OS 2 019 324, DE-OS 2 314 512 and also Angew. Chem 82, 53 (1970)].
The dispersibility of the polyisocyanate polyaddition products present in these dispersions is based on the presence of incorporated ionic centers, especially incorporated sulfonate or carboxylate groups. In the production of coatings from these dispersions, the ionic centers generally remain in the resulting coatings, which causes a reduction in the water resistance of the coating.
Water-dispersible polyisocyanate addition products modified by anionic cyanourea groups are also known (cf. DE-0S 3 441 934; 3 600 595; 3 735 198 and 3 813 840). Coatings obtained from dispersions such as these are considerably more water-resistant than coatings obtained from standard PUR dispersions because, after losing the counterion, the hydrophilicizing cyanourea anions acquire a self-crosslinking character and thus lose their hydrophilic properties after crosslinking. In addition, dispersions of the type in question can also be crosslinked with other crosslinking agents, for example polyepoxides.
A disadvantage is that the incorporation of the hydrophilicizing cyanourea groups in accordance with the previously described German references is a chain-terminating reaction. As a result, the molecular weight which the polyurethanes are capable of reaching is limited by the number of cyanourea groups required for dispersion. In extreme cases, the effect of this can be that the molecular weight capable of being reached in the case of very fine particle dispersions requiring a high content of hydrophilicizing groups is so low that the products are no "longer film-forming. However, fine particle PUR dispersions are advantageous in terms of processing because they have good flow and, with the proper molecular weight, are characterized by excellent film formation.
By contrast, synthesis components for incorporating lateral, hydrophilic carboxylate and/or sulfonate groups into the polyurethanes or polyurethane-ureas are strictly difunctional in the context of polyurethane chemistry. Therefore, the amount of hydrophilic groups incorporated does not affect the molecular weight buildup of the polymer. The direct consequence of this is that virtually any desired molecular weight can be obtained. The hydrophilicizing groups are laterally incorporated into the polyurethanes of the type in question.
An object of the present invention is to provide anionically modified polyureas which combine the advantages of the two types of hydrophilic polyurethanes and polyurethaneureas previously described, i.e., 1) the use of hydrophilic components which crosslink spontaneously after drying to avoid the problems caused by the presence of hydrophilic groups in the resulting coating and 2) the use of hydrophilic components which may be incorporated without affecting molecular weight buildup to avoid the difficulties associated therewith.
This object may be achieved in accordance with the present invention as described hereinafter.